1. Field of the Invention
The present invention is broadly concerned with abatement of pollutants, particularly with adsorbable pollutant concentration in gaseous streams. More specifically, the present invention is concerned with a novel apparatus, method, and system for concentrating adsorbable pollutants (such as NOx and SOx) present in the exhaust stream of internal combustion engines (gasoline or diesel) comprising at least two adsorbable pollutant adsorption means (trapping materials) capable of operating simultaneously and in parallel to adsorb and concentrate the pollutants. The concentrated pollutants then may be further treated to form non-polluting effluents.
2. Related Art
It is well known in the art of internal combustion engine exhaust treatment to utilize catalyst compositions such as those commonly referred to as three way conversion (TWC) catalysts. Such catalysts have been found to successfully promote both oxidation of unburned hydrocarbons (HC) and carbon monoxide (CO) and the reduction of NOx in the exhaust gas, provided that the engine is operated at or close to stoichiometric air/fuel conditions. However, many present day engines, especially gasoline-fueled engines used for passenger automobiles and the like, are designed not to operate at stoichiometric air/fuel conditions at least part of time as a fuel economy measure. In such lean-operating engines, the ratio of air to fuel in the combustion mixture supplied to the engine is maintained above the stoichiometric ratio so that the resulting exhaust gases are "lean", i.e., the exhaust gases are relatively high in oxygen (5-10% O.sub.2) content and relatively low in reductants content, e.g., HC, CO, and/or hydrogen.
Although lean-burn engines provide enhanced fuel economy, a disadvantage is that conventional TWC catalysts cannot adequately abate the NOx component of the pollutants in the gas stream because high NOx conversion rates require reductant-rich conditions. The art has devoted great effort to treat the exhaust of lean-burn engines. The best catalysts known to abate NOx from engines under lean-burn conditions are those containing platinum. However, these catalysts have limitations of only reducing about 20 to 40% of the NOx in the exhaust mainly to N.sub.2 O over a relatively narrow temperature range of 200-275.degree. C. Another disadvantage of such catalysts is the high oxidation activity of SO.sub.2 to sulfate under lean-burn conditions.
Other efforts to reduce NOx in lean-burn engines include combinations of TWC catalyst systems with NOx traps (sorbent materials). The NOx traps contain sorbent materials capable of adsorbing or "trapping" oxides of nitrogen and sulfur under lean conditions. The trapped oxides are desorbed and reduced under fuel-rich conditions thereby regenerating the traps. Therefore, it has been proposed that these so called NOx traps be utilized in what is called a "partial lean-burn" operating strategy. Here, the vehicle operates lean under some driving conditions and is controlled at the stoichiometric air-to-fuel (A/F or .lambda.) ratio under other driving conditions. While operating lean, the engine is periodically subjected to rich fuel or reductant spikes to desorb and reduce NOx and regenerate the trapping function. The problem with this operating mode is that the vehicle cannot take full advantage of the fuel economy gains obtained under full-lean burn operating conditions because the engine operates lean only a fraction of the time. In addition, the rich spikes result in significant fuel economy losses because a significant amount of reductant is required to create a rich condition in the exhaust. Therefore, the gains obtained from the lean operating modes are less than 100%.
U.S. Pat. No. 5,365,734 to Takeshima discloses an apparatus having at least two lean NOx catalysts installed in parallel in the exhaust gas system of an internal combustion engine. The apparatus has means for changing the space velocity of the exhaust gas to each of the catalysts such that when the space velocity through one catalyst is high, the space velocity through the other catalyst is low. It is stated that by alternating the exhaust gas between high and low space velocities across the NOx catalyst, improved NOx catalysis is achievable. The NOx catalyst is said to be an ion-exchanged transition metal/zeolite catalyst. Examples of transition metals given are copper and cobalt. However, the patent is silent with regards to means for adsorbing adsorbable pollutants such as NOx and SOx to provide a concentrated pollutant-containing stream.
The present invention provides an advance over the related art by providing a novel adsorbable pollutant concentration solution as described herein.